Java is a programming language designed by Sun Microsystems and offers many benefits to the professional programmer and application developer. Java is a byte-compiled language and is completely portable. You can run the same Java program on a wide range of operating systems. Java is often faster than interpreted languages (e.g. TCL, Perl) but slower than fully compiled languages (C, C++).

Because of its portability, Java and the World Wide Web make an excellent match. With a Java-enabled browser, Web designers can embed applets into their Web content. The applets are downloaded over the Internet with the context of the Web document and are then executed on the local computer. Applets can add interactivity, animations, multimedia, or database interfaces to an otherwise dull and listless Web site.

Programming with the Java Virtual Machine

The Java Virtual Machine is at the heart of the Java programming language. In fact, you cannot run a Java class or Java applet without also running an implementation of the Java Virtual Machine. Internet browsers Netscape and MSIE each include an implementation of the Java Virtual Machine (usually referred to as a Java runtime environment or JRE).

The Java Virtual Machine is the engine that actually executes a Java program. When a Java program is run, the instructions are not executed directly by the hardware of the local system, instead an interpreter or "virtual processor" walks through the instructions step-by-step and implements the action the instruction represents. This may seem abstract, but it actually provides a level of protection between your computer and the software you run on your computer. With a Java Virtual Machine, it is very easy to insert protections that prevent a program from performing malicious acts, such as deleting files on your disk or corrupting memory.

Using Java on the VPS v2 Virtual Server

There are several Java tools currently available that are compatible with version 1.0.2 of the Java specification. The 1.0.2 specification is supported by all Java-enabled browsers. Java 1.1.8 is installed on your VPS v2 Virtual Server.

Decide whether to set the filepath where Java needs to look in order to run, on a global basis or on an individual basis.

(Global)

(Individual using csh, tcsh, or zsh)

Move to the end of the “set path” line and type:

(Individual using sh, bash, or ksh)

Move to the end of the “set path” line and type:

Java tools

Java tools on your server are the Java Bytecode compiler and Java Virtual Machine with the “just in time” (JIT) compiler.

Java Bytecode Compiler (javac)

javac converts Java source code (.java files) into .class files that contain the Java bytecode for the class. For example:

where Test.java is a Java source code file. The resulting class file can then be embedded into Web content. If you have a Java-enabled browser, you can check out the example applet yourself.

Java Virtual Machine (Interpreter) and "Just-in-Time" Compiler (java)

The Java Virtual Machine is an interpreter for Java bytecode. This also includes a "Just-In-Time" (JIT) code generator. JIT is a technique for speeding up the execution of interpreted programs. The idea is that, just before a method is run for the first time, the machine-independent Java bytecode for the method is converted into native machine code which can then be executed by the computer directly. The JIT code generator greatly increases the speed of interpreted bytecode to nearly the speed of compiled code. For example:

This executes the Test.class bytecode compiled with the javac bytecode compiler (see above). (The Java Virtual Machine installed on the servers is java_x 1.18.)